A conventional hydraulic clutch actuating system for motor vehicles has a master cylinder connected with an equalizing reservoir filled with hydraulic fluid. The master cylinder has a cylinder housing with a housing bore in which a piston is received to be longitudinally displaceable. The master cylinder piston bounds a pressure chamber in the housing bore of the cylinder housing and can be mechanically loaded with an actuating force by a clutch pedal or an electric-motor drive. The pressure chamber of the master cylinder is hydraulically connected with a pressure chamber of a slave cylinder by way of a pressure line so that the pressure generated in the pressure chamber of the master cylinder by depressing the clutch pedal or electric-motor displacement of the master cylinder piston is transmissible to the pressure chamber of the slave cylinder by way of a fluid column in the pressure line. The slave cylinder similarly includes a cylinder housing with a housing bore in which a piston bounding the pressure chamber of the slave cylinder and thus loadable with the pressure generated in the master cylinder is received to be longitudinally displaceable. Associated with the slave cylinder piston is a piston rod engaging a clutch lever which in turn is in operative connection with the release bearing of the clutch. As a result, the release bearing of the clutch can be loaded with an actuating force via the slave cylinder piston so as to separate the clutch pressure plate from the clutch drive disc by way of a release mechanism and thus the motor from the transmission of the motor vehicle.
The slave cylinder commonly has a spring element serving the purpose of biasing the slave cylinder piston in a direction away from a base of the cylinder housing so as to keep the end of the piston rod in the mounted state of the slave cylinder in contact with the associated seat at the clutch lever and to exert a preload on the release bearing. In previously known solutions, this spring element is primarily constructed as a helical compression spring. See, for example, U.S. Pat. No. 7,216,752 B2 (FIG. 1) when the spring element in a first alternative is arranged in the pressure chamber where it is supported at one end on the base of the cylinder housing and at the other end on the piston by way of a spring plate. Conversely, see, for example, U.S. Pat. No. 7,287,376 B2 (FIG. 1) where the spring element is at the side of the piston opposite from the pressure chamber, surrounds the piston rod, and at an end thereof distant from the piston bears against an annular collar of the piston rod. The spring element is supported at the end thereof near the piston on the cylinder housing by way of a spring plate.
The latter alternative is advantageous insofar as there is no risk of the spring element contacting and possibly damaging the cylinder housing wall in the region of the seal sliding surface. Moreover, in this alternative of the slave cylinder, self-bleeding is better not only in the case of first filling, but also in operation of the slave cylinder, because there is no spring element, to which air bubbles could adhere, in the pressure chamber. Finally, in this alternative the spring element advantageously also serves for setting the piston rod straight relative to the cylinder housing, which facilitates assembly of the slave cylinder.
In connection with, in particular, hydraulic clutch actuating systems which are supplied to the assembly line of the motor vehicle manufacturer as a unit preassembled from master cylinder, equalizing reservoir, pressure line and slave cylinder and filled with hydraulic fluid, measures have been proposed to temporarily shackle the piston rod of the slave cylinder with respect to the cylinder housing of the slave cylinder so as to avoid overfilling of the hydraulic clutch actuating system and for transport thereof as well as to facilitate mounting of the unit in the motor vehicle.
The slave cylinder according to U.S. Pat. No. 7,287,376 B2, thus has a fixing section which fixes the piston subassembly of the piston and piston rod in a predetermined stroke setting with respect to the cylinder housing against the force of the spring element prior to first actuation of the slave cylinder and is constructed to release the piston subassembly relative to the cylinder housing when first actuation of the slave cylinder takes place. The fixing section is provided in the pressure chamber at the housing base and co-operates with a mating element at the piston for releasable fixing of the piston subassembly with respect to the cylinder housing. More precisely, the fixing section is formed at an insert member inserted into a pressure connection, which is provided at the housing base, of the slave cylinder. A piston extension at the pressure chamber side of the piston forms a mating element able to be mechanically positively detented with the fixing section of the insert member.
This arrangement of the fixing section in the pressure chamber at the housing base of the cylinder housing is certainly advantageous to the extent that the end of the piston rod remote, i.e., opposite from the piston remains free for non-problematic mounting of sliding socket, protective dust cap or the like, and the fixing section does not obstruct angular movement of the piston rod with respect to the center axis of the cylinder housing and cannot cause undesired friction noise. However, additional installation space is needed in the pressure chamber of the prior art slave cylinder in order to accommodate the fixing section and this space lengthens the slave cylinder for a given stroke, which can be undesirable for specific confined installation situations.
What is needed is a hydraulic cylinder for a hydraulic actuating system for motor vehicles, in which the piston subassembly can be fixed in a predetermined stroke setting relative to the cylinder housing prior to the first actuation of the hydraulic cylinder and can be released when first actuation of the hydraulic cylinder takes place, without the measures for temporary fixing of the piston subassembly requiring additional installation space.